activating voicemail on iphone 7

My 93 year old mother didn't want to use voicemail on her iphone 7 when we originally purchased it for her. Now she wants to use it. Contacted ATT, our provider, and they activated voicemail on her iphone 7 but it requires calling your voicemail phone number and using it as if you were on your landline voicemail...Press this, press that?? Do we need to download a fix from Apple to get the standard iphone voicemail on the phone instead?

Thank you for any help with this issue...